Various toners are being used as the electrophotographic toner, however, almost all are amorphous toners prepared by the so-called grinding method and a very small number of toners are spherical toners prepared by the polymerization method.
In recent years, the toner is required to satisfy various demands so as to improve the image quality or save energy of a copying machine or printer. For example, the toner is keenly required to have a small particle size for improving the image quality, however, the amorphous toner prepared by the grinding method cannot be free of conspicuous reduction in the fluidity as a result of formation into small particles. Further, the colorant comes out on the surface of a toner particle and accordingly, the control of electrostatic charge is troublesome. Furthermore, in order to reduce the demand power of a copying machine or printer, the toner must have both the capability of fixing at low temperatures and the hot-offset resistance. Almost all toners (in particular, a negatively polar toner comprising a styrene-acrylic resin as the binder) use a CCA (charge control agent), however, the CCA contains a heavy metal and may pollute the environment and therefore, it is desired, if possible, not to use the CCA.
The electrophotographic toners known up to the present time in the field of conventional techniques such as the grinding method or polymerization method have not succeeded in satisfying all these requirements.
The process for preparing a toner by the phase inversion emulsification method is a new technique of which basic technique is first disclosed in JP-A-5-66600 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"). JP-A-5-66600 describes a negatively charging toner comprising an anionic self-emulsifiable binder resin having encapsulated therein a colorant and the preparation process thereof, in which the particle size distribution and the triboelectricity are mainly studied but the fixability is not specifically compared and studied. The binder resin used here is a non-crosslinked styrene-acrylic resin having a weight-average molecular weight of from 35,000 to 60,000. The styrene-acrylic resin comprises nearly 4% of the components having a molecular weight in terms of polystyrene by GPC of 200,000 or more and has a Mw/Mn ratio of about 3.
To speak roughly, the preparation process of the toner by the phase inversion emulsification method is a technique of dispersing an organic solvent solution of a resin containing a hydrophilic group and having a self-emulsifiability, a colorant and the like in an aqueous medium and forming a spherical particle having encapsulated therein a colorant.
This process is advantageous in that the particles can be easily prepared without consuming huge energy such as grinding and at the same time, small size particles can be very easily prepared. Further, particles having a sharp grain size distribution can also be prepared. Furthermore, since a surface active agent or protective colloid is not substantially used, the cleaning operation can be simple and the problem of environmental stability of the charging is relatively lightened, as compared with the process using a dispersion stabilizer such as a polymerization toner.
However, in the preparation process of the toner by the conventional phase inversion emulsification method, an organic solvent solution of resin is used and a crosslinked resin cannot be used substantially, since the crosslinked resin is not dissolved in the organic solvent. Further, even if the resin used is not crosslinked, when the resin used contains nearly 4% of the components having a molecular weight in terms of polystyrene by GPC of 200,000 or more and has a Mw/Mn ratio of about 3 as described above, hot-offset generating temperature is low and it is difficult to achieve satisfactory fixing properties.
Furthermore, the toner prepared by the phase inversion emulsification method is disadvantageous in that since the toner particle contains a polar group having hygroscopicity, the environmental stability (the degree of deflection in the triboelectricity of toner upon changing of the temperature or humidity) is insufficient and also it is considered to be ascribable to the spherical shape of the toner particle, the charge rising at the triboelectric charging is deficient and this causes a problem on practical use.
JP-A-6-258869 discloses a toner comprising as essential components a resin having a crosslinkable site, a crosslinking agent for the resin and a colorant and a process for the preparation thereof. This toner undergoes crosslinking reaction when acted upon by a heat supplied by a heating roller during heat fixing to exhibit an improved fixability. However, this approach is disadvantageous in that the crosslinking reaction proceeds gradually even during the storage of the toner. Thus, this toner lacks of storage stability. Further, the toner composition unavoidably becomes unstable when used. Thus, this toner lacks of quality stability. This toner has another great disadvantage that if the fixing speed is raised, the amount of heat required for crosslinking reaction runs short, unavoidably providing insufficient fixing.
In the present invention, a toner particle is prepared by forming a spherical particulate comprising a resin having a crosslinkable site, a crosslinking agent for the resin and a colorant as essential components, and then allowing the resin to undergo crosslinking reaction in the presence of the crosslinking agent to completion. The resulting toner can be provided with a sufficient storage stability and quality stability as well as a drastically improved heat roll fixability.
A toner comprising a crosslinked binder resin is disclosed in U.S. Pat. Nos. 5,489,498, 5,500,318, 5,338,638 and 5,180,649. However, these citations concern a toner substantially obtained by crushing process, i.e., amorphous toner having a colorant, wax, etc. exposed on the surface thereof. The present invention concerns a spherical toner comprising a styrene-acrylic resin having an acid value falling within a specific range as a binder resin, at least part thereof being crosslinked, and a colorant, wax, etc. in capsulized form. There is a big difference in basic properties of toner such as triboelectricity and powder fluidity between a spherical toner comprising a colorant, etc. in capsulized form and an amorphous toner having a colorant, etc. exposed on the surface thereof.
The ultimate object to be attained by the present invention in an optimized embodiment is to provide a toner which (1) is excellent in the heated roll fixability, (2) has good fluidity even when the particle is formed to have a small size and (3) exhibits good triboelectricity even without using a CCA, more specifically, to provide a spherical toner capable of fixing with a heated roll over a wide fixing temperature range and having excellent triboelectricity while improving the thermal fixability and the environmental stability of charging which are the problems most earnestly desired to solve in the conventional phase inversion emulsification method.